Welding torch oscillation apparatus

ABSTRACT

Apparatus for rapid and automatic welding of joints which interconnect sections of pipe, and in particular oil or gas transmission pipe. The system includes at least one torch transport assembly which simultaneously moves a plural number of welding torches along a path parallel to the pipe joint being welded. The torch transport assembly includes track segments which surround circumferential portions of the pipe joint, with a welding torch carrier and multiple torches being independently movable along each track segment. The torch transport assembly is mounted on a support apparatus which is rigidly clamped to the pipe sections during welding, and which permits the torch transport assembly to be rapidly positioned in rough alignment with a pipe joint. The torch transport assembly and selected operating parameters of each welding torch are adjusted to accurately position each torch with respect to the pipe joint, and to repeatably provide programmed welding parameters resulting in a uniform pipe joint weld. The present system is disclosed in the context of hot-wire gas-tungsten arc welding torches, and in the operating environment of out-of-position joint welding.

This is a division of application Ser. No. 654,975, filed Feb. 3, 1976now U.S. Pat. No. 4,145,593.

This invention relates in general to welding and in particular tooff-axis welding of pipeline in which a welding torch is traversed withrespect to a relatively stationary workpiece.

Transmission pipelines are frequently used to transport fluid productsfor substantial distances, with oil and gas transmission pipelines beingbut two well-known examples. Such pipelines are generally constructed ofindividual pipe sections that are joined together by welding, and it isimportant that each welded pipe joint meet the criteria necessary forsafe operation of the pipeline. Not only is the detection and repair ofdefective joints an extremely expensive undertaking, particularly wherethe pipeline is buried underground or submerged beneath the sea, but theexistence of a defective weld joint creates a hazard of catastrophe inthe case of pipelines which carry flammable products.

The separate lengths of pipe which make up an oil or a gas transmissionpipeline are typically interconnected by electric-arc welding, and manytypes of welding techniques are used or proposed in the art for thatpurpose. Perhaps the simplest welding technique is shielded metal arcwelding, commonly known as "stick welding," in which one or more personsmanually weld each joint using hand-held welding torches of conventionaldesign. Since each welded joint of a typical pipeline actually consistsof several separate weld segments or layers, known as "passes," manualwelding techniques are time-consuming and costly in view of the largenumber of welded pipe joints to be welded in a pipeline extending formany miles. The relative slowness of manual and other conventionalwelding techniques is particularly troublesome in pipe laying operationswhere wages and equipment expenses provide a costly overhead which mustbe rationialized by maximizing the number of acceptable welds that canbe produced per unit of time.

Lay-down operations for underwater pipeline typically present an extremeexample of operating overhead costs, since all pipe welding and relatedoperations must take place on a barge or other vessel with maintenancecosts in addition to the costs of crew and equipment directly related tolaying the pipeline. Such barges are subjected to the wave motion of theprevailing sea state, moreover, so that even relatively massive bargesfrequently present a moving work surface relative to the completedportion of pipeline which extends from the barge into the sea. It hasbeen found that the pipeline cannot be rigidly clamped to the deck ofthe lay-down barge, since sea-induced movements of the barge can readilydamage the pipeline. Such problems further complicate the task ofwelding sections of pipe to the completed portion of the pipeline.

Welding devices have been proposed which traverse a welding torch aboutthe circumference of a pipe joint while welding the joint. Since it isobviously impossible to rotate the joint end of a pipeline which mayextend for many miles from a laydown barge (or any other joint weldingsite), automatic pipe welding equipment generally traverse a weldingtorch circumferentially about the pipe joint. Those skilled in the artwill recognize that "off-axis" welding, that is, welding applicationswhere the welding torch departs from an upright vertical position,becomes increasingly difficult as the welding torch departs the 12o'clock or upright position in its passage around the pipe joint. Theweld puddle is subjected to natural forces including gravity, surfacetension, and capillary attraction within the grooved joint being welded,and the net force acting on the puddle constantly changes as the torchtraverses about the circumference of a pipe joint which is in anonhorizontal plane. When the torch is welding at the 6 o'clockposition, maximum care must be exercised to prevent the weld puddle fromfalling out of the weld by gravity.

It has been proposed to overcome gravitational pullout of the weldpuddle during off-axis welding by applying pulsed welding current to thetorch, so that the weld puddle will slightly congeal during each "off"portion of the pulsed weld current. The operating speed of weldingtorches receiving pulsed welding current must be correspondinglyreduced, however, and it is possible that the welds which are producedby the pulse-induced intermittent partial cooling may have undesirablemetallurgical properties.

The speed at which a pipejoint can be welded is determined by themaximum rate at which the welding torch can deposit weld metal whiletraversing a pipe joint which is in a nonhorizontal plane. While gasmetallic arc (MIG ) welding torches generally have a relatively highrate of metal deposition, such torches generally produce welds that arenonuniform and difficult to repeatably obtain. Gas tungsten arc (TIG)welding torches are known to produce a pipe joint weld of superior andmore repeatable quality, although the metal deposition rate for TIGwelding torches is relatively slow. A development known as the hot-wireTIG torch, in which electric current is passed through the filler wireto preheat the filler wire which is melted in the weld puddle, is knownto produce a substantially increased rate of metal deposition, relativeto conventional TIG welding. The lack of sufficiently precise andrepeatable weld parameter control of prior-art hot-wire TIG weldingtorches, however, along with the aforementioned problem of off-axis weldpuddle control and related problems, have heretofore kept themetal-deposition rates of hot-wire TIG torches from being fully realizedin pipeline welding applications. A description of hot-wire TIG weldingis set forth in U.S. Pat. No. 3,122,629.

Accordingly, it is an object of the present invention to provide animproved apparatus for welding along a predetermined path of workpiecessuch as pipe joints or the like.

It is another object of the present invention to provide apparatus forautomatically welding pipe joints and the like, at an improved rate ofmetal deposition and with an improved quality of weld.

It is another object of the present invention to provide a pipelinewelding system in which precise alignment of one or more welding torchescan be accomplished with only rough positioning of one or more torchcarriers in relation to a pipe joint being welded.

Still another object of the present invention is to provide aprogrammable welding system in which selected welding parameters areautomatically varied in relation to welding torch position or otherfactors.

Stated in general terms, the present invention comprises a pipe-engagingsupport apparatus which positively engages two sections of pipe adjacenta common joint; a welding torch transport means which is supported bythe support means, and which is movable relative to the support meansfor positioning in proximity to the pipe joint; and control means whichcontrols selected operating parameters of one or more welding torchesassociated with the torch transport means. Stated somewhat morespecifically, the torch transport means is connected to the supportmeans for movement along several degrees of freedom relative to thesupport means and the pipeline sections engaged by the support means, sothat welding torches associated with the torch transport means aremaintained in fixed relation with the pipeline irrespective of pipelinemovements relative to a barge deck or other off-pipeline work station.The torch transport means includes track sections which are parallel tothe joint, and carriages which are movable along each track. Eachcarriage supports one or more welding torches in angular offsetrelation, relative to the circumference of the pipe joint to be welded.Each carriage as well as each welding torch carried by the pluralcarriages, is independently operable to provide preselected optimalwelding parameters for the particular circumferential position of eachwelding torch relative to the circumferential pipe joint being welded.Automatic control of the arc length for each torch, as well aselectrical adjustment of the center point of weld-joint transverseoscillation for each torch, permits the torches to independentlymaintain a precision track about the pipe joint irrespective of possibleeccentricity of the support means and carriages with respect to the pipejoint.

The foregoing and other objects and advantages of the present inventionwill become more readily apparent from the disclosed preferredembodiment as described below with respect to the drawings, in which:

FIG. 1 is a detailed pictorial view of a welding head assembly used inthe disclosed embodiment;

FIG. 2 is a pictorial view, partially broken away, of the mechanismwhich provides automatic voltage control and oscillation of one of thewelding head assemblies;

FIG. 3 is a rear elevation view of the mechanism shown in FIG. 2;

FIG. 4 is a top plan view of the mechanism shown in FIG. 2;

FIG. 5 is a semi-schematic block diagram of the torch oscillationcontrol for one of the welding head control circuits of the presentinvention;

FIGS. 6A and 6B graphically depict two examples of torch oscillationcontrol provided by the present invention;

The illustrative embodiment of the present welding torch oscillatingapparatus includes a welding head assembly H1 which, in an actualembodiment of the present invention, is one of four such assemblies thatare supported by a carriage which moves along a predetermined path withrespect to a workpiece. Specific details of the carriage which supportsthe welding head assembly H1, along with associated control apparatus,are found in copending application Ser. No. 654,975 filed Feb. 3, 1976now U.S. Pat. No. 4,145,593. The details of a typical welding headassembly H1 are shown in FIGS. 1-4. The welding head assembly isconnected to the carriage by a pair of support rods 100a, 100b whichextend inwardly from the carriage and terminate in a bracket 101 towhich the head assembly is secured. The head assembly H1 carries a torchsupport 102 on which is mounted a welding torch assembly T. The torch Thas a torch base 147 which is connected to the torch support 102 by thebolt 148. The head assembly H1 is independently capable of moving thetorch support 102, along with the torch assembly T, along a verticalpath 103 which is vertically perpendicular to the axis of the pipe jointbeing welded, and along a horizontal path 104 which is horizontallyperpendicular to that joint. Movement along the vertical path varies thearc spacing between the torch assembly T and the joint to be welded asthe torch assembly T is moved by its supporting carriage, and thevertical path movement is operated by the automatic voltage controldescribed in the aforementioned copending application, while movementalong the horizontal path 104 oscillates the torch assembly with respectto the joint.

The torch support 102 is mounted at the outer ends of a pair of rods105a, 105b which extend outwardly from a vertically-elongated opening106 in the front wall 107 of the welding head assembly H1. Each of therods 105a and 105b slidably extends through corresponding openings inthe front face 110a of the bearing block 110 and extends inwardly to beslidably received through mating openings in the rear face 111 of thebearing block. The bearing block is connected to the vertical travel bar112, which is supported by the vertical shafts 113a, 113b and thecorresponding ball bushings 114a, 114b for vertical reciprocatingmovement along the path 103. The vertical travel bar 112 has a nut 115which engages the vertical rotary screw 116. The screw 116 is driven ineither direction by the stepping servomotor 117 and the meshing gears118. It will be understood that the vertical travel bar 112 verticallyslides along walls 119a, 119b and, along with the rods 105a and 105bwhich carry the torch support 102, is moved up or down depending on therotational direction of the motor 117.

The two rods 105a and 105b extend through openings in the rod clampingblock 122 and are positively retained therein by the tightened clampscrew 123. The clamp block 122 slidably extends through the elongatedhorizontal slot 121 in the bearing block 110 between the front wall 110aand the rear wall 111, and also slidably extends through the verticalslot 124 in the horizontally oscillating travel bar 125. The verticalextent of the slot 124 is at least equivalent to the movement range ofthe vertical travel bar 112, so that the clamp block 122 is free to movevertically within the slot 124 as the vertical travel bar and the block110 are moved by the motor 117.

The oscillating travel bar 125 is connected to the linear ball bushing128 which is supported for reciprocal movement along the support shaft129. Clamped to the ball bushing 128 at 128a for oscillation therewithis the coil 132 of a linear variable differential transformer (LVDT),which moves horizontally with respect to the LVDT core 133 as thebushing 128 moves horizontally.

Connected to the oscillating travel bar 125 is a nut 136 which engagesthe horizontal screw 137. The screw 137 is connected by gearing 138 tothe stepping servomotor 139, so that the travel bar 125 moves along thehorizontal path 104 to an extent and in a direction determined byoperation of the motor 139. Clamp block 122 is carried with the travelbar 125 to slide within the horizontal slot 121 in the block 110. Sincethe rods 105a and 105b are secured to the clamp block 122, the torchsupport 102 is moved horizontally by operation of the motor 139 and thehorizontal position of the torch assembly T is sensed by the output ofthe LVDT coil 132. It will be understood that the output of the LVDT islinearly proportional to the displacement of the coil 132 relative tothe core 133, with the coil output being zero volts when centered on thecore 133 and with the voltage increasing positively with horizontalmovement of the coil in one direction and negatively with coil movementin the other direction.

The welding torch holder H1 receives operating signals from theoscillation control 233, and it should be understood that theoscillation control in turn receives control command signals from asuitable source of programmed and/or manual position controls such asdisclosed, for example, in the aforementioned copending patentapplication.

A block diagram of the oscillation control 233 is shown in FIG. 5, andthe operation of the oscillation control is better understood byreference to the position control waveforms of FIGS. 6A and 6B. Theoscillation control 233 operates the oscillator servomotor 139-1 of therelated welding head assembly H1 to move the torch assembly across thepipe joint at a constant velocity, and at a rate and width of movementwhich are independently adjustable. The left and right limits of travelare determined electronically, without external limit switches or motionstops, and the welding head assembly is capable of remaining at the leftand/or right motion limits for independently-adjustable periods of dwelltime. The centerpoint of oscillation across the weld joint isadjustable, so that the torch assembly can be "centered" directly abovethe joint by adjusting a control such as a potentiometer or the like,before initiating a particular weld sequence.

As mentioned above, the LVDT coil 132 senses the position of theoscillating torch assembly, to provide an output voltage which islinearly proportional to the displacement of the torch assembly. TheLVDT preferably provides zero volts output when the torch assembly is atthe mechanical center of its maximum oscillation width, with outputvoltage going positive in one direction of movement from mechanicalcenter and good negative with movement in the other direction. Referringto FIG. 6A, the zero-volt center line represents the mechanical centerof oscillation, with positive-voltage departures representing horizontalmovement to the right of center and with negative voltage representingmovement to the left of center. Where the electrically-set center ofoscillation corresponds to the mechanical center of oscillation, asillustrated in FIG. 6A, the center is represented by zero volts and thewidth of oscillation is represented by two signals of equal voltage andopposite polarity. Thus, the right limit is represented by 3 volts andthe left limit is represented by -3 volts. The slope of the movementline 398 corresponds to the rate of horizontal movement, and the rightor left limits of oscillation are determined when position voltage fromthe LVDT corresponds to the preset aforementioned right or left limitvoltages. The right dwell and left dwell periods are denoted on FIG. 6A.

Turning next to FIG. 6B, it is there assumed that the torch assembly hasbeen centered over a welding joint by displacement from the mechanicalcenter, represented by the zero-volt line 399, to the 3-volt line 400corresponding to a certain displacement to the right of mechanicalcenter. The width of torch assembly movement shown in FIG. 6B is thesame as the width in FIG. 6A, namely, horizontal movement represented by±3 volts of LVDT movement with respect to the offset 3-volt "centerline" 400.

Turning to FIG. 5, the oscillator servomotor 139-1 receives a positionfeedback input along the line 404 from the LVDT, a center set signalalong the line 405 from a suitable source of variable voltage such as apotentiometer 406 connected across a suitable voltage source, and aposition command signal on line 407. The position feedback signal online 404 and the center set signal on line 405 are also supplied to thecenter comparator 408, which senses when the torch position signal online 404 is equal to the center set signal on line 405 and provides anoutput signal on line 409 when such signal equality occurs. The outputon line 409 is supplied to the coincidence gate 412, which may alsoreceive an oscillation stop signal along line 413 from the a source ofcarriage control signals, such as disclosed in the aforementionedcopending application. If the stop signal is present on the line 413when the torch assembly moves through a "center" position correspondingto the center set potentiometer 406, the coincidence gate 412 provides asignal on line 414 to clear and hold the integrator circuit 415, asdescribed below, thereby stopping the torch assembly at the centerposition determined by potentiometer 406. An example of such centersignal coincidence is shown at 416 in FIG. 6B.

The rate of torch assembly oscillation is determined by the oscillationrate control circuit 418, which provides alternative rate commandsignals of equal voltage but opposite polarity on the line 419, as aninput signal to the integrator circuit 415. The magnitude of the ratecommand signals provided by the rate control circuit 418 is determinedby the rate set input signal on line 420, from an appropriate source,such as disclosed in the aforementioned copending application. Thepolarity of the rate command signal on line 419 is controlled inflip-flop manner by signals received on the lines 421 or 422, asdescribed below.

The oscillation width signal, as previously mentioned, is represented bytwo signals of equal voltage and opposite polarity, and such widthsignals are provided by the oscillator width control circuit 426. Thewidth signal, the amplitude of which is determined by the input on theline 427 from an appropriate source, is supplied along the lines 428 asa width command signal to the limiting circuit 429, and along the lines430 to provide an input to the right limit comparator 431 and the leftlimit comparator 432. Each of the right and left limit comparators alsoreceive the oscillator position input signal from line 404, and thecenter set signal from line 405.

The right limit comparator 431 compares the oscillator position signalwith a limit signal which is equivalent to the sum of the width signal(from the width control circuit 426) and the center set signal. Theright limit comparator 431 provides an output signal on line 435 to theright dwell timer 436 when the oscillator position signal becomes equalto the sum of the center set signal and the positive-polarity widthsignal, while the left limit comparator 432 similarly provides an outputsignal on the line 437 to the left dwell timer 438 when the oscillatorposition signal reaches the sum of the center set signal and thenegative-polarity width signal. Each of the right and left dwell timers436 and 438 provides an output signal, an adjustable predetermined timeafter receiving an input signal from the corresponding limit comparator,along the respective lines 422 and 421 to the rate control circuit 418.The polarity of the rate command signal is reversed whenever the ratecontrol circuit 418 receives a signal from either dwell timer.

Considering the operation of the oscillation circuit 233 shown in FIG.5, it will be understood that the fixed-amplitude rate command signal online 419 is integrated by the integrator circuit 415 (absent a stopsignal on line 413) so that a linear ramp signal is present on line 440from the integrator circuit. The ramp signal is applied through thelimiting circuit 429 to the oscillation servomotor 139-1 to operate theservomotor at a constant speed for traversing the torch assembly acrossthe pipe joint. The slope of the ramp signal on the line 440, andtherefore the rate of oscillation, is represented by the movement line398 on FIG. 6A, for example.

The limiting circuit 429 clamps the ramp output signal of the integratorcircuit 415 to the positive and negative width command voltages whichare alternatively present on the lines 428. Referring again to FIG. 6A,when the ramp voltage reaches the -3 volt left limit width commandsignal, the limiting circuit 429 clamps the ramp voltage to that levelas shown at 441, and the oscillation servomotor 139-1 stops to maintainthe torch assembly at the left limit of oscillation. Attainment of thisleft-limit condition is also sensed by the left limit comparator 432,which activates the left dwell timer 438. When the left dwell timer hastimed out, the polarity of the rate command signal applied on line 419from rate control circuit 418 is reversed, and the integrator circuit415 commences producing a ramp signal 442 (FIG. 6A) of opposite sloperelative to the ramp 398. The oscillation servomotor 139-1 thuscommences to move the torch assembly back across the joint toward theright limit, which is determined by the positive-polarity width commandsignal and the limiting circuit in a similar manner. The torch assemblycontinues to oscillate in the foregoing manner until an appropriateoscillation stop signal is applied along line 413 followed by occurrenceof a sensed center signal along the line 409, whereupon a signal isapplied along line 414 to interrupt operation of the integrator circuit415 at a time when the torch assembly is at the center set positiondetermined by the potentiometer 406.

It should be understood that the disclosed use of the present inventionto weld a vertical-plane joint about horizontal pipe is by way ofexample only, since the present invention is readily adaptable forautomatic welding along any path in response to preprogrammed weldparameters chosen for the weld path.

It will be understood that the foregoing relates only to a preferredembodiment of the present invention, and that numerous changes andmodifications may be made therein without departing from the spirit andthe scope of the invention as set forth in the following claims.

We claim:
 1. Apparatus for oscillating a welding torch across a weldpath, comprising:means for supporting a welding torch in relation to aweld path; motive means operatively connected to move said torch supportmeans laterally of the weld path in a selected first or second directionand at a selected rate of movement; position sensing means responsive tothe lateral position of said torch support means to provide a locationsignal corresponding to said lateral torch position; position controlmeans responsive to an input command signal to provide said motive meanswith position command signals which cause said torch support means tomove alternately in first and second lateral directions at apredetermined rate; center adjustment means operative to provide aselectably variable center signal corresponding to a selectably variablecenter location of said torch support means at a selected location onthe path of said lateral movement; center coincidence means responsiveto said selectively variable center signal and to said location signalfrom said position sensing means to provide a center coincidence signalin response to signals indicating that said torch support means is atsaid selected center location; and center stop means operative inresponse to the occurrence of said center coincidence signal in thepresence of an oscillation stop signal to interrupt said positioncommand signal and to maintain said center signal, so that said torchsupport means stops at said variable center location on said path oflateral movement in response to said oscillation stop signal. 2.Apparatus as in claim 1, wherein said position control means isoperative to provide said position command signal which is variable withrespect to time, so as to command said lateral movement of said torchsupport means; andsaid center stop means includes means operative inresponse to said center coincidence signal and said oscillation stopsignal to control said position control means to terminate said timevariable position command signal and to maintain said center signal, sothat commanded lateral movement of said torch support means stops atsaid selected center location.
 3. Apparatus as in claim 2, wherein:saidposition control means comprises means for integrating a signal toprovide said time variant position command signal to which said motivemeans is responsive; and said integrating means is responsive to saidcenter stop means to remove said integrated signal from said motivemeans.